One type of memory known in the art is referred to as a resistive cross point memory (RXPtM). Memory cells in an RXPtM provide resistance values that correspond to logic states, such as logic “0” or logic “1”. One exemplary type of RXPtM is a magnetic random access memory (MRAM). An MRAM is a non-volatile memory that includes magnetic memory cells.
A typical magnetic memory cell includes a layer of magnetic film in which the magnetization of the magnetic film is alterable and a layer of magnetic film in which the magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is referred to as a sense layer, and the magnetic film that is pinned, is referred to as a reference layer.
A typical magnetic memory includes an array of magnetic memory cells. Word lines extend along rows of the magnetic memory cells, and bit lines extend along columns of the magnetic memory cells. Each magnetic memory cell is located at an intersection of a word line and a bit line. A magnetic memory cell is written to a logic state by applying magnetic fields that rotate the orientation of magnetization in the sense layer. The logic state of a magnetic memory cell is indicated by the resistance through the memory cell, which depends on the relative orientations of magnetization in the sense layer and reference layer.
A read circuit is used to sense the resistance state of a selected magnetic memory cell to determine the logic state stored in the memory cell. The resistance state can be sensed by applying a voltage to a selected memory cell and measuring a sense current that flows through the memory cell. The resistance is proportional to the sense current.
Sensing the resistance through a memory cell in the array can be unreliable. The memory cells in the array are coupled together through many parallel paths. The resistance at one cross point equals the resistance of the memory cell at that cross point in parallel with the resistances of memory cells in the other word lines and bit lines. Memory cells located along the same word line or bit line typically each see similar resistances.
Read circuits are calibrated to reduce the effect of parasitic resistances. Long read circuit calibration times may interfere with read and write operations in the memory. Hence, it is important for the read circuit to be calibrated and for the calibration times to be relatively short.